1. Field of the Invention
This invention relates to a method and an apparatus for recording various forms of information such as imagery and characters. The invention particularly relates to a technology for producing high-quality records using a recording head having a plurality of recording elements.
2. Description of the Related Art
FIG. 16 shows a general layout of a related art recording apparatus comprising a rotating recording drum and a recording head. In the recording apparatus indicated by 100, a generally cylindrical recording drum 110 is rotatably supported on a frame and a recording medium 112 can be fixed onto the outer circumference of the drum 110. The direction in which the drum 100 rotates corresponds to the main scanning direction.
A recording head 114 is fixed on a moving stage 116 such that it is capable of sliding along the guide member of the stage 116. The direction in which the recording head 114 moves along the stage 116 corresponds to an sub-scanning direction. The recording head 114 is typically composed of a plurality of laser diodes (LD) so that it is capable of issuing a plurality of laser beams.
As shown in FIG. 17, the recording medium 112 consists of a colorant or toner sheet 10 and an image-receiving sheet 12. The toner sheet 10 consists of a base 11 coated with a light-heat converting layer 14 and a toner layer 15 on the side which faces the image-receiving sheet. The image-receiving sheet 12 consists of a base 16 coated with a cushion layer 17 and an image-receiving layer 18 on the side which faces the toner sheet.
The base 11 is made of a material that transmits laser beam, as exemplified by PET (polyethylene terephthalate), TAC (triacetyl cellulose) or PEN (polyethylene naphthalate). The light-heat converting layer 14 may be made of carbon, a black substance, an IR absorbing dye, a substance capable of absorbing specific wavelength, or any other substances that are capable of efficient conversion of laser energy to heat. The toner layer 15 is typically for black (K), cyan (C), magenta (M) and yellow (Y) colors and it may be for special colors such as gold, silver, brown, gray and green. In a recording mode, the toner layer 15 is brought into intimate contact with the image-receiving layer 18 in the image-receiving sheet 12 and, upon illumination with laser beams from the recording head 114, it is heated to have the toner image transferred to the image-receiving layer 18 in the image-receiving sheet 12.
The base 16 may be the same as the base 11 of the toner sheet 10 or it may be a light-opaque base. In a recording mode, the image-receiving layer 18 is brought into intimate contact with the toner layer 15 in the toner sheet 10 and the heated areas of the toner layer 15 are transferred to the image-receiving layer 18. When transfer is made from a plurality of toner sheets 10 in superposition, the cushion layer 17 effectively absorbs the difference in toner size.
FIG. 18 shows the steps in a process of forming a color image consisting of K, C, M and Y colors. Lamination, if performed at all, precedes laser recording for each color.
1) The image-receiving sheet 12 is wrapped onto the outer circumference of the recording drum.
2) A K toner sheet 10 is wrapped onto the image-receiving sheet 12.
3) Laser beams are applied in accordance with the data for K imagery and characters so that a record in K color is formed on the image-receiving sheet 12.
4) The K toner sheet 10 is stripped from the image-receiving sheet 12.
5) A C toner sheet is wrapped onto the image-receiving sheet 12.
6) Laser beams are applied in accordance with the data for C imagery and characters so that a record in C color is formed on the image-receiving sheet 12.
7) The C toner sheet is stripped from the image-receiving sheet 12.
8) A M toner sheet is wrapped onto the image-receiving sheet 12.
9) Laser beams are applied in accordance with the data for M imagery and characters so that a record in M color is formed on the image-receiving sheet 12.
10) The M toner sheet is stripped from the image-receiving sheet 12.
11) A Y toner sheet is wrapped onto the image-receiving sheet 12.
12) Laser beams are applied in accordance with the data for Y imagery and characters so that a record in Y color is formed on the image-receiving sheet 12.
13) The Y toner sheet is stripped from the image-receiving sheet 12.
14) As the result of steps 1)–13), K, C, M and Y colors are formed in a predetermined pattern of superposition to produce the desired color image.
15) The color image is then transferred to a final recording sheet.
When lamination is to be performed with a view to assuring better adhesion in records, the thermal transfer sheet is compressed under a pressure roller, a heated roller or the like just before recording each color with laser and this allows the thermal transfer sheet to be brought into intimate contact with the image-receiving sheet.
To perform laser recording in the manner described above, laser beam spots are arranged as typically shown in FIG. 19. The pattern shown in FIG. 19 consists of three columns of spots in the main scanning direction and five rows of spots in the sub-scanning direction, totaling to 15 spots (spot No. 1 to spot No. 15).
This arrangement of spots is herein called “a basic spot arrangement”. The spot in the first column as counted from the right end and which is in the first row as counted from the bottom is called the first spot; the other spots in the first column are called the second spot, the third spot, the fourth spot and so forth. The basic spot arrangement as used herein is such that even if no recording elements are mounted in the respective spot positions, the individual spots are assigned numbers according to the system just described above. When recording is done by flashing all laser beam spots simultaneously in the main scanning direction according to the basic spot arrangement, a solid image is formed as shown in FIG. 20, in which the recorded areas are hatched.
In the case of recording with a plurality of spots with toner sheets of different colors being superposed sequentially on the image-receiving sheet on the recording drum, the same spot channel is used for the same place (i.e., the same recording line in the sub-scanning direction). In the case of K color shown in FIG. 21, the line number in the sub-scanning line coincides with the spot channel number and this is also true with the case of C color shown in FIG. 22, as well as with Y and M colors.
However, if all spots are fired simultaneously to record the solid image shown in FIG. 20, the outside of the two-dimensional spot array is greatly influenced by the surrounding ambient temperature and the spot positions at both ends are comparatively cooler than the inside spot positions. To be more specific, every inside spot position that is located between adjacent spots is hotter than the surrounding ambient temperature and, hence, hotter than the spot positions at both ends. This means that the lines recorded with the spots at both ends are comparatively thin whereas the lines recorded with inside spots are comparatively thick. As a result, the recorded image can potentially have a small gap that is created by every rotation of the recording drum as shown in FIG. 23 (to be more exact, at intervals of 15 lines).
A further problem occurs if all spots are fired simultaneously to record the solid image shown in FIG. 20. Exposure first starts at each preceding spot position (channels 1, 6 and 11), so these preceding spots are cooler than the other spot positions (channels 2–5, channels 7–10 and channels 12–15). To be more specific, these other spot positions are hotter than the preceding spot positions on account of the heat from the adjacent channels that first started exposure. In other words, the lines recorded with the preceding spots are comparatively thin and the lines recorded with the other spots are comparatively thick. As a result, the recorded image can potentially have a small gap that is created for every column in the two-dimensional spot array as shown in FIG. 24 (to be more exact, at intervals of 5 lines).
Thus, recording a plurality of colors in the same place (the same recording line in the sub-scanning direction) has the disadvantage of exaggerating the characteristics inherent in channels to such an extent that they are highly visible as image defects. From the viewpoint of spots, the characteristics inherent in channels include power, spot size, spot shape and wavelength and from the viewpoint of recording characteristics, they include line width, jaggedness at line edges and density. Among these, line width is greatly influenced by the characteristics of thin spot channels and thick spot channels. To be more specific, a gap may form between adjacent thin spot channels whereas the image recorded by a thick spot channel may spread to the adjacent channels, with the result that the density intended by the image data is not attained or the dots in the adjacent channels which should be left blank are not actually left blank. In each of these cases, unevenness occurs in the recorded image.